Reverse transcriptase (RT) and p30 from several retroviruses have been shown to bind together specifically to form a high molecular weight complex. This complex displays increased DNA-polymerse activity over that shown by RT alone. Such a complex was detected in the thymocytes and spleenocytes of AKR mice prior to the onset of disease. The amont of the complex is maximum when the clinical symptoms manifest. The complex can be seen only in the target organs. In the animals treated with either antiviral IgG or poly(A-U) or interferon which delayed the development of spontaneous leukemia, the complex appeared much later in life but prior to the development of disease. Further characterization of the complex suggests that p30 of the complex is more related to xenotropic virus.